The spinal column is a highly complex system of bones and connective tissues that provides support for the body and protects the delicate spinal cord and nerves. The spinal column includes a series of stacked vertebral bodies, each vertebral body including an inner or central portion of relatively weak cancellous bone and an outer portion of relatively strong cortical bone. Situated between each vertebral body is an intervertebral disc that cushions and dampens compressive forces exerted upon the spinal column. A vertebral canal containing the spinal cord and nerves is located behind the vertebral bodies.
A surgical technique commonly referred to as spinal fixation uses surgical implants for fusing together and/or mechanically immobilizing two or more vertebral bodies of the spinal column. Spinal fixation may also be used to alter the alignment of adjacent vertebral bodies relative to one another to change the overall alignment of the spinal column. Such techniques have been used effectively to treat a wide variety of conditions and, in most cases, to relieve pain.
One spinal fixation technique involves the fusion of adjacent bone structures. Conventional procedures for a fusion procedure include partial or total excision of an injured disc portion, e.g., discectomy, and replacement of the excised disc with biologically acceptable plugs or bone wedges. The plugs are placed between adjacent vertebrae to maintain normal intervertebral spacing and to achieve, over a period of time, bony ingrowth or “fusion” with the plug and opposed vertebrae.
Alternatively, a fusion cage may be inserted within a tapped bore or channel formed in the intervertebral space to stabilize the vertebrae and maintain a pre-defined intervertebral space. A pair of fusion cages may also be implanted within the intervertebral space. After a period of time, the soft cancellous bone of the surrounding vertebral bone structures infiltrates the cage through a series of apertures in the cage wall and unites with bone growth inducing substances disposed within an internal cavity of the cage wall to eventually form a solid fusion of the adjacent vertebrae.
Presently existing fusion cages are sized to fit between adjacent vertebrae by cutting the cage to adjust the length of the cage. The length of the cage may also be adjusted by providing end caps, the position of which can be adjusted to alter the overall length of the cage. For example, U.S. Pat. No. 6,344,057 describes a cylindrical fusion implant that has an adjustable length in that threaded end caps can be adjusted telescopically with respect to the cage. With regards to cutting fusion cages, there does not appear to be any convenient method and apparatus for quickly and accurately measuring and cutting the length of a fusion cage. Typically, a surgeon will use a caliper or other measuring device to determine the appropriate length of the spinal implant to fit in an intervertebral space, but there is no convenient way to transfer this measurement to a cutting device to make an accurate cut based on the intervertebral spacing measured by the caliper. It would be desirable to provide improved apparatus and methods for measuring and cutting spinal implants such as fusion cages to a desired length.